I am trying to build a spectrometer to measure the color intensity of a solution.
By chemistry I can make up a solution that has exactly 1 mg of gold in it and another that has 5 mg of gold in it. If I extract each of these into 15ml of solvent I will get a solution of known color and volume. I will have two knowns, each with a different color intensity. These color intensities increase uniformly over the small increments taken and I can determine the exact weight of a similar prepared unknown sample, this is called a chemical assay.
I built my unit to measure this color intensity using a 2" solar cell from a solar pathway light, a multimeter, and a LED light source. It is all enclosed so the multimeter reads zero with the light off.
I have the multimeter set on 2000 mV. When I shine my LED light thru my sample solution vial I get a reading of 1600 mV for the 1 mg and 1400 mV for the 5mg. (Remember more = color less light.) That would be perfect, a 200 mV difference, or 50 mV per mg of gold.
When the light was held on the sample for a few seconds the readings started to go up and down one or two digits at a time and varied over 25 mV up and down. A ball breaker!
I tried a laser which did not work at all, and then tried a flash light with an incandescent bulb, did not work either. I also took the unit out into direct sun light and it still jumped around.
When I had the unit apart at my desk testing it with different lights, I noticed that my desk light was shining on it and it was giving me a stable reading. This lamp is a 60 watt incandescent soft light.
I also read where solar cells like halogen lights. I was trying to make this unit portable so I can do assays in the field so having a 60 watt, 110 volt light source is not practical. I was looking for other lighting (10 volt max) or different solar cell.